In general, in the descriptions that follow, we will italicize the first occurrence of each special term of art that should be familiar to those of ordinary skill in the art of low power current reference design. In addition, when we first introduce a term that we believe to be new or that we will use in a context that we believe to be new, we will bold the term and provide the definition that we intend to apply to that term. In addition, throughout this description, we will sometimes use the terms assert and negate when referring to the rendering of a signal, signal flag, status bit, or similar apparatus into its logically true or logically false state, respectively, and the term toggle to indicate the logical inversion of a signal from one logical state to the other. Alternatively, we may refer to the mutually exclusive boolean states as logic_0 and logic_1. Of course, as is well known, consistent system operation can be obtained by reversing the logic sense of all such signals, such that signals described herein as logically true become logically false and vice versa. Furthermore, it is of no relevance in such systems which specific voltage levels are selected to represent each of the logic states.
Power consumption has become a key problem for circuit designers with the proliferation of battery-powered devices. Circuit topologies that support power reduction are extremely valuable in extending battery life. Reference current generators are present in virtually any integrated circuit (“IC”) since all analog electronics require a bias current for proper operation. This reference current is also generally temperature-compensated such that the current is substantially insensitive to temperature or proportional to absolute temperature (“PTAT”) or complementary to absolute temperature (“CTAT”). Most reference current generators draw significant power due to the heavy use of saturated transistors and relatively small resistors.
Reference currents can be generated in a wide variety of ways. Several prior art examples are shown in FIG. 1 and FIG. 2. In one such prior art example of a reference current generator circuit 10, shown in FIG. 1, an amplifier 12 develops a reference current, IRef, proportional to a reference voltage, VRef, developed by a reference voltage generator (e.g., a bandgap reference voltage generator) 14 across a resistor 16. Reference voltage generator 14 and resistor 16 are both reasonably temperature insensitive, and can be tuned to achieve a desired temperature sensitivity (e.g., zero temperature sensitivity, PTAT, CTAT). However, reference current generator circuit 10 consumes considerable power, and, in particular, reference voltage generator 14 draws significant power, nominally on the order of one microamp (1 μA). Further, the combination of a large reference current, IRef, combined with a relatively small resistor 16 results in additional power dissipation. Assuming, e.g., a typical bandgap reference voltage of 1.25V and a typical on-chip resistor 16 of 100 kΩ, reference current generator circuit 10 consumes a reference current of 1.25/100e3=12.5 μA. This current is well in excess of limits imposed by many modern battery-powered devices.
Shown in FIG. 2 is an alternative circuit 18 that can achieve good temperature sensitivity. However, the active devices in reference current generator circuit 18 are operated in the saturation region, and thus will typically draw much more than 1 μA of current.
FIG. 3 illustrates a reference current generator 20 we first disclosed to the University of Michigan, and which is now the subject matter of the Related Applications. This reference current generator 20 is capable of creating a carefully controlled current reference in an energy efficient manner, as disclosed in the Related Application. The generated current reference can be substantially insensitive to temperature, PTAT, or CTAT. Generally speaking, the reference current generator 20 operates by generating a pair of voltages, illustrated in FIG. 3 as VTOP and VBOT. This pair of voltages may be each buffered and amplified. An exemplary buffering circuit is illustrated in FIG. 3; VTOP is buffered and amplified by a two stage buffer comprising a first stage amplifier circuit 22 and a second stage N-channel transistor 24, resulting in the substantially constant voltage at the source of the N-channel transistor 24, illustrated as VTOPBUF. A similar configuration of first stage amplifier 26 and P-channel transistor 28 operates to buffer VBOT in an analogous manner. The resulting pair of buffered and amplified voltages may then be applied across resistive element 30. FIG. 3 also illustrates the current generating half of a typical current mirror comprising a diode-connected P-channel transistor as the source of the desired reference current, illustrated as IREF in the figure. Detailed disclosure of these circuits is contained and illustrated in the Related Applications and should be readily apparent to one of ordinary skill in this art. FIG. 4 illustrates an example where P-channel transistors may readily be used in lieu of N-channel transistors for the voltage ladder. FIG. 4 also illustrates the current generating half of a typical current mirror as N-channel transistor 32′, illustrating that an N-channel current mirror may readily be used in lieu of its P-channel transistor equivalent. These circuit configurations will be readily apparent to one of ordinary skill in this art. The specific implementation disclosed in the Related Applications is of high value due to its energy efficient qualities. However, this implementation lacks several elements that are critical to mass production of IC devices that utilize the specific implementation. In particular, the desired elements lacking are: (1) a mechanism for measuring the internal state, e.g., voltages and currents, after manufacture; (2) an adjustment mechanism for managing post-silicon variations due to process, voltage, and temperature variations in the manufacturing process; and (3) a mechanism for generating multiple currents and/or voltages for use elsewhere in the IC device.
Given the wide use of current reference generators and the significant power demands of these circuits, we submit that what is needed is an improved method and apparatus for an ultra-low power temperature compensated reference current generator that addresses the lack of the aforementioned elements. Such a method and apparatus are important for use in power sensitive systems such as battery-powered electronics.